With respect to the prior art related to the present invention, reference can be made to the DE Utility Model Publication 29601543, to the DE Patent 3910600, to the U.S. Pat. Nos. 5,515,619 and 4,856,205, from which publications a mechanical seal is known which is used in connection with a blow-suction box or equivalent, which is in contact with, or in a near proximity of, a wire or equivalent, and which seals the closing nip between the box and the adjacent cylinder in order to cut off a boundary-layer air flow. A boundary-layer air flow follows a paper web and a wire when they come from a drying cylinder towards a reversing cylinder or roll. In the prior-art solutions, the seal is mechanical and often also rigid, in which case it may damage the wire coming from the cylinder, or it can be broken itself. These physical seals can be broken as a result of prolonged operation, and, moreover, owing to deterioration or damaging of a seal, the capacity of blow-suction boxes is lowered considerably in paper machines.
One problem in the prior-art blow-suction boxes has been the problems of runnability as air is pumped through the wire in a nip, which results in separation of the web from the wire and, at the same time, causes wrinkles and bag formation.
One object of the present invention is to provide a solution in which the problems mentioned above have been eliminated or at least minimized.
With respect to the prior art, reference is also made to the applicant's FI Patent No. 80,491 (equivalent to U.S. Pat. No. 4,905,380), in which a method and a dryer group in a multi-cylinder dryer in a paper machine are described, in whose connection a blow-suction box is employed which includes a suction and/or closing compartment, by whose means the upper free sectors of reversing cylinders between the adjacent wedge spaces have been covered. In the gaps between drying cylinders, said combined blow-suction boxes are placed, which have a plane wall at the inlet side of the drying wire and of the web, into whose edge a nozzle opening or openings is/are opened, by whose means a flow is blown which ejects in the direction opposite to the direction of movement of the drying wire placed facing said nozzle opening(s), by means of which flow a field of vacuum is induced in the gap space between said wall and the straight run of the drying wire and the web as well as in the following wedge space. An object of the present invention is further development of the blow-suction box so that the effects of the boundary-layer air flow are eliminated and that the effect achieved by means of the blow-suction box on the support and runnability of the paper is improved.
As is known from the prior art, multi-cylinder dryers in paper machines usually consist of two horizontal rows of drying cylinders placed one row above the other, between which the paper web runs meandering. In connection with the drying cylinders, currently, drying wires permeable to air are employed. In the dryer section of a paper machine, single-wire draw and/or twin-wire draw is/are employed. Single-wire draw is understood as a draw in which the web runs from one drying cylinder onto the other on support of one and the same drying wire also between the rows of cylinders. In twin-wire draw, in which an upper wire and a lower wire are employed, the web has free unsupported draws when it runs from one row of cylinders to the other. The present invention is suitable for use most advantageously in applications with single-wire draw.